Perspective based green screening

ABSTRACT

A method of generating a composite image includes capturing a video image of a physical scene with a camera, identifying a green-screen region within the video image, identifying a viewpoint and a position and/or orientation of the green-screen region relative to the viewpoint, and generating a modified video image rendered from the viewpoint onto a display surface in which the green-screen region is replaced with an image of a virtual object. The image of the virtual object is generated by projection rendering of a model of the virtual object based on the position and/or orientation of the green-screen region relative to the viewpoint such that the virtual object is constrained within the green-screen region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 16/679,093, filed onNov. 8, 2019, which claims priority to U.S. Provisional Application No.62/757,604, filed on Nov. 8, 2018. The disclosures of the priorapplications are considered part of and are incorporated by reference inthe disclosure of this application.

BACKGROUND Technical Field

This disclosure relates to a three-dimensional display system, and inparticular, to a display process to render virtual objects in avisualized three dimensional space reflecting the data presented to therendering system.

Description of Related Art

Three dimensional (3D) capable electronics and computing hardwaredevices and real-time computer-generated 3D computer graphics have beena popular area of computer science for the past few decades, withinnovations in visual, audio, tactile and biofeedback systems. Much ofthe research in this area has produced hardware and software productsthat are specifically designed to generate greater realism and morenatural computer-human interfaces. These innovations have significantlyenhanced and simplified the end-user's computing experience.

Ever since humans began to communicate through pictures, they faced adilemma of how to accurately represent the three-dimensional world theylived in. Sculpture was used to successfully depict three-dimensionalobjects, but was not adequate to communicate spatial relationshipsbetween objects and within environments. To do this, early humansattempted to “flatten” what they saw around them onto two-dimensional,vertical planes (e.g., paintings, drawings, tapestries, etc.).

The two dimensional pictures must provide a numbers of cues of the thirddimension to the brain to create the illusion of three dimensionalimages. This effect of third dimension cues can be realisticallyachievable due to the fact that the brain is quite accustomed to it. Thethree dimensional real world is always and already converted into twodimensional (e.g., height and width) projected image at the retina, aconcave surface at the back of the eye. And from this two dimensionalimage, the brain, through experience and perception, generates the depthinformation to form the three dimension visual image from two types ofdepth cues: monocular (one eye perception) and binocular (two eyeperception). In general, binocular depth cues are innate and biologicalwhile monocular depth cues are learned and environmental.

A planar stereoscopic display, e.g., a LCD-based or a projection-baseddisplay, shows two images with disparity between them on the same planarsurface. By temporal and/or spatial multiplexing the stereoscopicimages, the display results in the left eye seeing one of thestereoscopic images and the right eye seeing the other one of thestereoscopic images. It is the disparity of the two images that resultsin viewers feeling that they are viewing three dimensional scenes withdepth information.

SUMMARY

In one aspect, a method of generating a composite image includescapturing a video image of a physical scene with a camera, identifying agreen-screen region within the video image, identifying a viewpoint anda position and/or orientation of the green-screen region relative to theviewpoint, and generating a modified video image rendered from theviewpoint onto a display surface in which the green-screen region isreplaced with an image of a virtual object. The image of the virtualobject is generated by projection rendering of a model of the virtualobject based on the position and/or orientation of the green-screenregion relative to the viewpoint such that the virtual object isconstrained within the green-screen region.

In other aspects, a computer program product or system is configured toperform the method.

Implementations may include one or more of the following features.

The video image is stereoscopic. The green-screen region may be asurface within the stereoscopic image. The green-screen region may be avolume within the stereoscopic image.

Identifying the green-screen region may include converting the image toa 3D model of the physical scene. Identifying the green-screen regionmay include detecting a color in the image. Identifying the green-screenregion may include detecting a shape in the image. Identifying thegreen-screen region may include data representing a three-dimensionalshape, receiving a signal indicating a position of an object within thephysical scene, and selecting a region corresponding to thethree-dimensional shape placed at the position.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present disclosure can be obtained whenthe following detailed description of the preferred embodiment isconsidered in conjunction with the following drawings, in which:

FIG. 1 presents a prior art display chain;

FIG. 2 presents a prior art polarization switch architecture;

FIG. 3 presents prior art left and right switching views causing astereo 3D effect;

FIG. 4A presents a prior art green screen system before green-screenprocessing;

FIG. 4B presents a prior art green screen system after green-screenprocessing;

FIG. 5A presents an example scene before image processing;

FIG. 5B presents the example scene after image processing;

FIG. 6 is a flow diagram of an example process for perspective-basedgreen screening.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical conventional display chain 10, whichincludes the following components:

1. Graphics Processing Unit (GPU). The GPU 12 typically resides on apersonal computer, workstation, or equivalent, and outputs video levelsfor each color or channel of a supported color model, e.g., for each ofthree colors, typically Red (R), Green (G), and Blue (B), for each pixelon the display. Each of these numbers is typically an 8 bit number, witha range of 0 to 255, although other ranges are possible.

2. Scaler. The scaler 14 is a video processor that converts videosignals from one display resolution to another. This component takes asinput the video levels (e.g., for R, G, and B) for each pixel outputfrom the GPU, and processes them in various ways, before outputting(usually) modified video levels for RGB in a format suitable for thepanel, usually in the same 8-bit range of 0-255. The conversion can be ascaling transformation, but can also possibly include a rotation orother linear or non-linear transformation. The transformation can alsobe based on a bias of some statistical or other influence. The scaler 14can be a component of a graphics card in the personal computer,workstation, etc.

3. Panel. The panel 16 is the display screen itself. In someimplementations, the panel 16 can be a liquid crystal display (LCD)screen. In some other implementations, the panel 16 can be a componentof eyewear that a user can wear. Other display screens are possible.

Time Sequential Stereo Displays

Unlike a normal display, in a stereo display, there are two images—rightand left. The right image is to be delivered to only the right eye, andthe left image is to be delivered to only the left eye. In a timesequential stereo display, this separation of right and left images isperformed in time, and thus, it must contain some time-dependent elementwhich separates these two images. There are two common architectures.

The first architecture, shown in FIG. 2 , uses a device called apolarization switch (PS) 20 which may be a distinct (separate) orintegrated LC device or other technology switch. The polarization switch20 is placed in front of the display panel 24, specifically between thedisplay panel 24 and the viewer. The display panel 24 can be an LCDpanel which can be backlit by a backlight unit 26, or any other type ofimaging panel, e.g., an organic light emitting diode (OLED) panel, aplasma display, etc., or any other pixelated panel display used in atime-sequential stereo imaging system. The purpose of the polarizationswitch 20 is to switch the light between two orthogonal polarizationstates. For example, one of these states may be horizontally linearlypolarized light (horizontal linear polarization state), and the othermay be vertically linearly polarized light (vertical linear polarizationstate); however, other options are possible, e.g., left and rightcircular polarization states, etc., the key feature being that the twopolarization states are orthogonal.

This allows achievement of the stereo effect shown in FIG. 3 . As may beseen, the top portion of the figure shows the (display) panel switchingbetween a left image and a right image. Synchronous with this, the PS isswitching between a Left State and a Right State. These states emit twoorthogonal polarization states, as mentioned above. The stereo eyewearis designed such that the left lens will only pass the Left Statepolarization and the right lens will only pass the Right Statepolarization. In this way, separation of the right and left images isachieved.

The second conventional architecture uses stereo shutter glasses, whichreplace the PS and eyewear. In this system, each eye is covered by anoptical shutter, which can be either open or closed. Each of theseshutters is opened and closed synchronously with the panel display insuch a way that when the left image is shown on the display, only theleft eye shutter is open, and when the right image is shown on thedisplay, only the right eye shutter is open. In this manner, the leftand right views are presented to the user's left and right eyes,respectively.

Prior Art Green Screening

Green screening, also called “chroma key compositing” or “chromakeying,” is a visual effects technique for compositing two images orvideo streams together based on a specified color hue. A color range ina foreground image is made transparent, allowing a background image tobe inserted into the scene depicted by the foreground footage.

Chroma keying can be performed with backgrounds of any color that areuniform and distinct, but green and blue backgrounds are more commonlyused because they differ most distinctly in hue from most human skincolors. If the subject being filmed or photographed has the same coloras the color being used for the chroma keying, then the subject will bemade transparent and lost.

In some implementations, the chroma-keyed area can outline a region inthe camera frame that is not the same color. Then, the chroma-keyed areaand the region in the camera frame that the chroma-keyed area outlinesare both rendered as transparent.

FIG. 4A shows a first example frame 400 before the frame is processedusing the chroma keying technique. The first frame 400 depicts a greenscreen 420 and a human 430. FIG. 4B shows a second example frame 410.The second frame 410 is the output of a system that processes the firstframe 400 using the chroma keying technique. The human 430 is notchanged, as the human 430 does not depict any color in the color rangethat the system used in the chroma keying process. The green screen 420of FIG. 4A was processed to generate a new screen 440 in FIG. 4B thatdepicts a different scene. The different scene depicted on the newscreen 440 is the portion of a background image that is visible beneaththe region of the screen 420 in FIG. 4A that was made transparent duringthe chroma keying process.

Similar to the color-based chroma keying technique is a shape-basedchroma keying technique, where an image processor recognizes aparticular shape in a camera frame instead of a particular color. Whenthe shape is recognized, the portion of the camera frame that depictsthe shape is rendered transparent, allowing separate background footage,computer graphics, or a static image to be inserted into the scenedepicted in the camera frame.

Terms

The following is a list of terms used in the present application:

Memory—may include non-transitory computer readable media, includingvolatile memory, such as a random access memory (RAM) module, andnon-volatile memory, such as a flash memory unit, a read-only memory(ROM), or a magnetic or optical disk drive, or any other type of memoryunit or combination thereof. Memory is configured to store any softwareprograms, operating system, drivers, and the like, that facilitateoperation of display system, including software applications, renderingengine, spawning module, and touch module.

Display—may include the display surface or surfaces or display planes ofany technically feasible display device or system type, including butnot limited to the display surface of a light-emitting diode (LED)display, a digital light (DLP) or other projection displays, a liquidcrystal display (LCD), optical light emitting diode display (OLED),laser-phosphor display (LPD) and/or a stereo 3D display all arranged asa single stand alone display, head mounted display or as a single ormulti-screen tiled array of displays. Display sizes may range fromsmaller handheld or head mounted display devices to full wall displays,which may or may not include an array of display devices. The displaymay include a single camera within a mono display device or a dualcamera for a stereo display device. The camera system is particularlyenvisioned on a portable display device, with a handheld, head mounted,or glasses device. The camera(s) would be located within the displaydevice to peer out in the proximity of what the user of the displaydevice might see; that is, facing the opposite direction of the displaysurface,

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), television system, grid computing system, or otherdevice or combinations of devices. In general, the term “computersystem” can be broadly defined to encompass any device (or combinationof devices) having at least one processor that executes instructionsfrom a Memory.

Viewpoint—This term has the full extent of its ordinary meaning in thefield of computer graphics/cameras and specifies a location and/ororientation. For example, the term “viewpoint” may refer to a singlepoint of view (e.g., for a single eye) or a pair of points of view(e.g., for a pair of eyes). Thus, viewpoint may refer to the view from asingle eye, or 25 may refer to the two points of view from a pair ofeyes. A “single viewpoint” may specify that the viewpoint refers to onlya single point of view and a “paired viewpoint” or “stereoscopicviewpoint” may specify that the viewpoint refers to two points of view(and not one).

Position—the location or coordinates of an object (either virtual orreal). For example, position may include x, y, and z coordinates withina defined space. The position may be relative or absolute, as desired.Position may also include yaw, pitch, and roll information, e.g., whendefining the orientation of a viewpoint and/or object within a scene orthe scene itself.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Graphical Processing Unit—refers to a component that may reside on apersonal computer, workstation, or equivalent, and outputs video levelsfor each color or channel of a supported color model, e.g., for each ofthree colors, typically Red (R), Green (G), and Blue (B), for each pixelon the display. Each of these numbers is typically an 8 bit number, witha range of 0 to 255, although other ranges are possible.

Functional Unit (or Processing Element)—refers to various elements orcombinations of elements. Processing elements include, for example,circuits such as an ASIC (Application Specific Integrated Circuit),portions or circuits of individual processor cores, entire processorcores, individual processors, programmable hardware devices such as afield programmable gate array (FPGA), and/or larger portions of systemsthat include multiple processors, as well as any combinations thereof.

Projection—refers the display of a 3D object, or content, on a twodimensional (2D) display. Thus, a projection may be described as themathematical function applied to objects within a virtual 3D scene todetermine the virtual position, size, and orientation of the objectswithin a 3D space that may be defined by the size of the 3D stereoscopicdisplay and the perspective of a user.

Concurrent—refers to parallel execution or performance, where tasks,processes, or programs are performed in an at least partiallyoverlapping manner. For example, concurrency may be implemented using“strong” or strict parallelism, where tasks are performed (at leastpartially) in parallel on respective computational elements, or using“weak parallelism”, where the tasks are performed in an interleavedmanner, e.g., by time multiplexing of execution threads.

Configured To—various units, circuits, or other components may bedescribed or claimed as “configured to” perform a task or tasks. In suchcontexts, “configured to” is used to connote structure by indicatingthat the units/circuits/components include structure (e.g., circuitry)that performs those task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. First, Second, etc. —these terms are used as labels fornouns that they precede, and do not imply any type of ordering (e.g.,spatial, temporal, logical, etc.). For example, in a system havingmultiple tracking sensors (e.g., cameras), the terms “first” and“second” sensors may be used to refer to any two sensors. In otherwords, the “first” and “second” sensors are not limited to logicalsensors 0 and 1.

Based On—this term is used to describe one or more factors that affect adetermination. This term does not foreclose additional factors that mayaffect a determination. That is, a determination may be solely based onthose factors or based, at least in part, on those factors. Consider thephrase “determine A based on B.” While B may be a factor that affectsthe determination of A, such a phrase does not foreclose thedetermination of A from also being based on C. In other instances, A maybe determined based solely on B.

Exemplary System

FIG. 5A illustrates an exemplary system that may be configured toperform various embodiments described below.

FIG. 5A shows an example physical scene 500. The physical scene includesa “green-screen” region 510 and a non-green-screen object 520. Images ofthe physical scene 500 can be captured by a first device 530 and/or asecond device 540. For example, the first device 530 and/or the seconddevice 540 can capture a still image of the scene 500 or a video of thescene 500 compose of multiple frames. The first device 530 and/or thesecond device 540 can also capture a laser or sonic scan of the scene500. A first initial image 531 a and second initial image 531 b,captured by the first device 530, and a third initial image 541,captured by the second device 540, depict the scene before therespective images are processed by a capture processing system 550 (inone implementation, an image processing system 550) to replace theregions of the image that identified the green-screen region 510 with asubstitute rendering of a virtual object. In this case, the virtualobject is a rendering of a three-dimensional dog.

As will be discussed in more detail below, the green-screen region 510can be a physical object, or more generally a region of the scene 500.In this case, the green-screen region 510 is an identified physicalobject, and so below the green-screen region 510 will also be referredto as a green-screen object 510.

In some implementations, one device can capture images of the scene 500,and a different device can display the images. That is, the first device530 can be composed of two different devices, one of which capturesimages and the other of which displays the images; the same is true ofthe second device 540.

The first device 530 and/or the second device 540 can send therespective initial captured images of scene 500 to the image processingsystem 550. In some implementations, the image processing system 550 canbe on-site, e.g. in the same room as the physical scene 500 and thedevices 530 and 540. In some other implementations, the image processingsystem 550 can be on the cloud, i.e., off-site. In some otherimplementations, the image processing system 550 can be a component ofthe first device 530 and/or the second device 540. In other words, eachof the devices can include a version of the image processing system 550,so that the initial images of the physical scene 500 can be processedon-device.

The green-screen object 510 can be identified in images of the scene 500by the image processing system 550.

For example, the surface of the green-screen object 510 can be aparticular color, e.g. green or blue. In this case, the image processingsystem 550 can store a predetermined color range for each color channelof the image, e.g., for each of the RGB channels. For the image fromeach device, those pixels in the image having values within the rangesare selected by the image processing system 550 as belonging to thegreen-screen object 510. In some implementations, the image processingsystem 550 can then determine a volume in virtual three-dimensionalspace that is enclosed by the selected pixels. For example, the imageprocessing system 550 can maintain a three-dimensional model of thescene 500 that includes coordinates for multiple objects in the scene500. The image processing system 550 can use the size and position ofthe green-screen object 510 in the image to determine its volume in themaintained model. This process is described in more detail below inreference to FIG. 6 . This volume then represents the three-dimensionalvolume over which the virtual object can be superimposed. In some otherimplementations, the image processing system 550 determines thetwo-dimensional size and shape of the green-screen object 510 in theimage, and superimposes a depiction of the virtual object on or inreplacement of the depiction of the green-screen objet 510 based on thetwo-dimensional size and shape.

As another example, the green-screen object 510 can have a particularshape that can be identified by the image processing system 550, e.g., acube or a cylinder. In this case, the image processing system 550 canapply a machine vision algorithm, e.g., a machine vision algorithmdeveloped through machine learning techniques, to identify theparticular shape within the image. The virtual object can besuperimposed over the identified shape according to a measured size ofthe shape in the image.

As another example, the green-screen object 510 can have a measured orpredetermined spatial location, shape, size, orientation, and/or volumerelative to each respective device 530 and 540. As a particular example,the green-screen object 510 can have a tracking component 515 that canbe used to track the location of the green-screen object 510 that may beassociated with a virtual coordinate system. In some implementations,the tracking component 515 can interact with a tracking base station560, which is a master tracking device that allows the location of everyobject in the scene 500 that has a tracker component to be determined.In some implementations, the tracking base station 560 determines thelocation of each object; in some other implementations, each objectdetermines its own location using the tracker base station 560. Ineither case, the location and orientation of the green-screen object 510can be determined continuously in real-time. This process is discussedin more detail below. In this case, the image processing system 550 canmaintain a three-dimensional model of the scene 500. The model caninclude the measured coordinates of the green-screen objects, e.g., alocation and orientation of the green-screen objects, as well as alocation and orientation of each of the capturing devices. The imageprocessing system 550 can infer the portions of the captured images thatdepict the green-screen objects using the relative locations andorientations of the devices and green-screen objects. This process isdiscussed in more detail below in reference to FIG. 6 .

In some implementations, the image processing system 550 renders anddisplays projections of a two-dimensional image in replacement to aparticular two-dimensional surface of the green-screen object identifiedin a captured image, instead of replacing the depiction of thegreen-screen object with a depiction of a virtual object. In some suchcases, the image processing system 550 can select a particular surfaceusing one of the methods discussed above, e.g., one of the surfaces ofthe green-screen object 510 has a particular color or a particularshape. In some other such cases, the image processing system 550 canidentify a green-screen object in the captured image using any of themethods discussed above, e.g., using a particular color or a particularshape. Then, the image processing system 550 can select one or more ofthe surfaces of the identified green-screen object using a set of one ormore criteria. As a particular example, the image processing system 550can identify a surface of the green-screen object that is closest to thedevice that captured the image, and project a two-dimensional image ontothis surface.

In some implementations, the green-screen region 510 is not an object,but rather is defined to be a particular region of space in the scene500, e.g., a three-dimensional portion of the scene 500 or atwo-dimensional portion of a wall in the scene 500.

In some such implementations, the image processing system 550 canidentify the particular region of the scene 500 in images of the scene500. For example, the image processing system 550 can maintain a datadefining the green-screen region 510 with respect to other objects inthe scene 500, e.g., values defining the height, width, and height of acubic region, as well as coordinates of the location and orientation ofthe region with respect to a wall in the scene or the non-green-screenobject 520. Then, the system 550 can identify the other objects in acaptured image of the scene 500, e.g., using a machine vision algorithmdeveloped through machine learning techniques, and then identify thegreen-screen region in the captured image in relation to the identifiedother objects.

In some other such implementations, the image processing system 550 canidentify the particular region of the scene 500 using a trackercomponent that is in the region. In this case, the system 550 canmaintain a three-dimensional model of the scene 500. The model caninclude the dimensions of the green-screen region, e.g., a height,width, and height of a cubic region or a green-screen region shape, aswell as the location and orientation of the region as measured by thetracker component; the model can also include a location and orientationof each of the capturing devices. The image processing system 550 caninfer the portions of the captured images that depict the green-screenregion using the relative determined locations and orientations of thedevices and green-screen region. For example, the image processingsystem 550 can store data indicating a predetermined position of theboundaries of the green-screen region 550 relative to the determinedlocation of the green-screen region. The image processing system canthen determine the viewpoint of a capturing device relative to themeasured location and orientation of the capturing device, and theninfer the position of the green-screen region within the captured imageof the scene.

The non-green-screen object 520 can be any captured physical object thatis not identified as a green-screen object by the image processingsystem 550. For example, if the green-screen object 510 is identifiedaccording to a particular color, then the non-green-screen object 520can have a different color than the green-screen object 510. As anotherexample, if the green-screen object 510 is identified according to aparticular shape, then the non-green-screen object 520 can have adifferent shape than the green-screen object 510. As another example, ifthe green-screen object 510 is identified according to a particularspatial shape, size, and/or location in a defined spatial region, thenthe non-green-screen object 520 can have a different shape, size, and/orlocation in a defined spatial region.

The first device 530 is a stereoscopic device, i.e., the first device530 captures images of the scene 500 and displays the images to a userin stereo; that is, the captured images are captured from two distinctperspectives, which are generally the perspective of each of the user'seyes. The first device 530 includes a first display 532 a and a seconddisplay 532 b. For example, the first device 530 can present the firstdisplay 532 a conveying the perspective captured from a the firstcapture device to the left eye of a user and the second display 532 bconveying the perspective captured from a the second capture device tothe right eye of a user, e.g., if the first device 530 is a head-mounteddisplay. The space between the first display 532 a and the seconddisplay 532 b can correlate to the separation between the two eyes ofthe user reflected by the capture from the two close, yet distinctperspectives.

The first device 530 includes a first camera 534 a and a second camera534 b. In some implementations, the first device 530 can have more thantwo cameras. The first camera 534 a and the second camera 534 b areseparated by a distance on the first device 530 so that the two camerascan capture the scene 500 in stereo, correlating to the two close, butdistinct perspectives. In some implementations, the separation betweenthe first camera 534 a and the second camera 534 b can correlate toapproximately the distance between the two eyes of the user. In someimplementations, the two cameras and the two displays have the samehorizontal relationship to each other, e.g., a line connecting the firstdisplay 532 a and the second display 532 b can be parallel to a lineconnecting the first camera 534 a and the second camera 534 b.

The first initial image 531 a depicts the scene 500 as it was capturedby the first camera 534 a. The first initial image 531 a would have beendisplayed to the left eye of the user on the first display 532 a if thefirst initial image 531 a were not to be processed by the imageprocessing system 550. The second initial image 531 b depicts the scene500 as it was captured by the second camera 534 b. The second initialimage 531 b would have been displayed to the right eye of the user onthe second display 532 b if the second initial image 531 b were not tobe processed by the image processing system 550.

The first device 530 can also include at tracker component 536, wherethe tracker component 536 that includes multiple photosensors. In someimplementations, the tracking base station 560 can determine thelocation of the first device 530 by interacting with the trackercomponent 536. In some other implementations, the first trackingcomponent 536 can determine its own location by interacting with thetracking base station 560. In either case, the location and orientationof the first device 530 can be determined continuously in real-time.This process is described in more detail below.

As a particular example, the first device 530 can be Skyzone SKY02S V+™3D virtual reality glasses with a stereo front facing camera and astereo rendering eyewear display. As another example, the first device530 can be a Rembrandt 3D™ tablet with a stereo front-facing camera anda stereo rendering display. In this case, the first display 532 a andthe second display 532 b are the same display.

The second device 540 is a handheld device with a single display 542,i.e., the display 542 is monoscopic instead of stereoscopic. The seconddevice 540 includes a camera 544 and a tracking component 546 thatperform similar functions to the cameras 534 a and 534 b and thetracking component 536 of the first device 530, respectively.

The third initial image 541 depicts the scene 500 as it was captured bythe camera 544. The third initial image 541 would have been displayed onthe display 542 if the third initial image 541 were not processed by theimage processing system 550.

For example, the second device 540 can be a smartphone or tablet runningthe Android, Windows, or iOS operations system that includes a displayon one side and one or more cameras on the other side.

While the first device 530 both captures and displays imagesstereoscopically, and the second device 540 both captures and displaysimages monoscopically, in general a stereoscopic capture device can havea monoscopic display and a monoscopic capture device can have astereoscopic display.

The tracking base station 560 allows the location and orientation ofeach object that has a respective tracking component to be tracked;e.g., the first device 530, the second device 540, and optionally thegreen-screen object 510 can be tracked using the tracking base station560. The tracking component of a given object can have multiplephotosensors that are separated by some distance.

In some implementations, the tracking base station 560 emits a radiationsignal, e.g., a wavelength of light or send. Each photosensor in thetracking component of a given object can reflect the radiation signalback to the tracking base station 560. The tracking base station 560 canuse the multiple returned radiation signals to determine the locationand orientation of the given object. For example, the tracking basestation can determine the 6 degrees of freedom of the object, e.g., thex-position, y-position, z-position, pitch, yaw, and roll of the objectaccording to a coordinate system. The tracking base station canrepeatedly perform this process in order to determine the location andorientation of the object continuously in real-time.

In some other implementations, the tracking base station 560 can emit afirst radiation signal and a second radiation signal concurrently, e.g.,if the tracking base station 560 includes two emitters that arephysically separated by a distance. Each photosensor in the trackingcomponent of a given object can detect the first radiation signal andthe second radiation signal at respective detection times, and thetracking component can use the respective detection times of each of thephotosensors to determine the position and orientation of the givenobject. As a particular example, the tracking base station 560 can be anHTC Vive Lighthouse™.

In some other implementations, the tracking base station 560 can includemultiple cameras capturing images of the scene 500. The tracking basestation 560 can perform object recognition on the captured images, andinfer the geometry of the respective tracked objects that are recognizedin the captured images.

Whether the position and orientation of each object is determined by thetracking base station 560 or by the object itself, the determinedposition and orientation can be provided to the image processing system550, along with the first initial image 531 a, the second initial image531 b, and the third initial image 541 of the scene 500.

In some implementations, a camera 570 can capture context images of thephysical scene 500 and send the context images to the image processingsystem 550. The context images can be used by the image processingsystem 550 when replacing the image region of green-screen object 510 ina captured image with the virtual object rendering to generate aprocessed image. The processed image that depicts the green-screenobject and/or region replaced with a virtual object and/or virtual sceneis also sometimes called a “composited” image. In some cases, thevirtual object projected rendering may not completely encompass thegreen-screen object 510 region in the initial image. That is, in someimplementation, when the green-screen object 510 is replaced by thevirtual object, portions of the scene 500 that are behind thegreen-screen object 510 relative to the device may be exposed, andshould be rendered in the processed image; that is, the virtual object510 or scene does not completely fill the green-screen object region. Asa particular example, the system might replace a spherical green-screenobject with a torus virtual object; in this case, the system needs torender the center of the torus with an image of what is behind thespherical green-screen object. In these cases, the context imagescaptured by the camera 570 can provide information about what is behindthe green-screen object 510, so that the exposed regions can be renderedrealistically in the processed image.

The image processing system 550 receives the location and orientation ofthe first device 530 and the second device 540, as well as the initialimages captured by the first device 530 and the second device 540, andoptionally the context images captured by the camera 570. The imageprocessing system 550 processes the initial images to replace thegreen-screen object 510 with the replacement projection rendering of thedog from a perspective corresponding to the perspective of each capturedevice. In particular, the image processing system 550 processes thefirst initial image 531 a to generate a first processed image 538 a,processes the second initial image 531 b to generate a second processedimage 538 b, and processed the third initial image 541 to generate athird processed image 548. This process is explained in more detailbelow in reference to FIG. 6 .

FIG. 5B shows the same scene 500 that was shown in FIG. 5A, after theimage processing system 550 has generated the three processed images andsent each processed image to the respective device: the first processedimage 538 a is displayed for the user on the first display 532 a of thefirst device 530, the second processed image 538 b is displayed for theuser on the second display 532 b of the first device 530, and the thirdprocessed image 548 is displayed for the user on the display 542 of thesecond device 540.

In each of the processed images, the green-screen object 510 has beenreplaced by a rendering of the dog. In particular, the image processingsystem 550 used a three-dimensional virtual object characterizing thedog to render each processed image according to the perspective positionand orientation of the camera that captured the respective initialimage. That is, the first processed image 538 a renders the dogprojection from the perspective point of view of the first camera 534 aof the first device, i.e., from the left side of the dog, with thenon-green-screen object 520 to the right of the dog. Similarly, thethird processed image 548 renders the dog from the point of view of thecamera 544 of the second device 540, i.e., from the right side of thedog, partially obscured by the non-green-screen object 520. Inparticular, the third processed image 548 shows parts of the rendereddog, e.g. the right side of the dog, that the first processed image 538a does not show, and vice versa.

The virtual object and/or virtual scene is constrained to be within thegreen-screen region, as depicted in each processed image. In otherwords, the depiction of the virtual object and/or virtual scene in aprocessed image cannot extend beyond the portion of the respectiveinitial image that depicted the green-screen object, but the virtualobject/scene projection is constrained to only render within thedetermined green-screen region and may not extend beyond that boundary.

The processed image can only extend beyond the portion of the respectiveinitial image that depicted the green-screen object, if the definitionof the green-screen region is defined as a regional function of theidentified green-screen region. In that condition the green-screenregion is expanded or contracted to that defined regional function ofthe identified green-screen region as predetermined by a user or set asa default within the image processing system. For example, thegreen-screen region can be defined as the region of space that extendsoutward in all directions from the green-screen object 510 a distanceequal to or approximately equal to 10% of the size of the green-screenregion. The determination of the green-screen region as determined bythe regional function is independent of the virtual object or virtualscene information.

The devices 530 and 540 and the image processing system 550 canrepeatedly perform this process in order to generate processed imagesfor each frame of a video, so that the user can view the processedimages as if they were being captured in real-time from the physicalscene 500. In particular, if the user moves the device around thegreen-screen object 510, the image processing system 550 willcontinuously generate images that depict the virtual dog from differentangles. Similarly, if the physical green-screen object 510 is movedwithin the scene 500, e.g., if the green-screen object 510 is movedfurther away from the non-green-screen object 520, then the imageprocessing system 550 will re-determine the correct position andorientation of the virtual object within each of the processed imagesaccordingly.

Exemplary Process

FIG. 6 is a flow diagram of an example process 600 for perspective-basedgreen screening. The process 600 allows a system to process an imagecontaining a green-screen object to replace the green-screen object witha depiction of a virtual object. For convenience, the process 600 willbe described as being performed by a system of one or more computerslocated in one or more locations. For example, an image processingsystem, e.g., the image-processing system 550 of FIGS. 5A and 5B,appropriately programmed in accordance with this specification, canperform the process 600. As another example, a stereoscopic ormonoscopic device, e.g., the first device 530 or the second device 540of FIGS. 5A and 5B, appropriately programmed in accordance with thisspecification, can perform the process 600. In some implementations, asubset of steps of the process 600 can be performed by animage-processing system that is separate from a stereoscopic ormonoscopic device, and the remaining steps of the process 600 can beperformed on-device by the respective stereoscopic or monoscopic device.

The system maintains a data characterizing a model of the virtual object(step 602). The model identifies a shape and size of the virtual object.The model can have associated data that identifies a position andorientation of the virtual object relative to a real-world physicalscene, where the real-world physical scene is defined by athree-dimensional coordinate system. The associated data can have sixdimensions in total: three dimensions defining the position of theobject and three dimensions defining the orientation of the object(e.g., pitch, yaw, and roll), where all dimensions are with respect tothe three-dimensional coordinate system of the real-world physicalscene.

Optionally, the system can maintain data characterizing the position ofone or more devices in the real-world physical scene (step 604). Thedevices can be monoscopic or stereoscopic devices, and can include acapturing component and a displaying component; i.e., the devices can beconfigured to capture and display images of the real-world physicalscene. For each tracked device, the system can maintain informationidentifying six degrees of freedom of the device in the scene, e.g.,three dimensions defining the position of the device and threedimensions defining the orientation of the device. The system canmaintain a time series of this information, where each element in thetime series characterizes the device at a different point in time. Insome implementations, the tracking information for a given device can beshared with other devices.

In some implementations, the system can also maintain datacharacterizing the location and orientation of one or more green screenobjects. In some such implementations, the system can track the locationof a green-screen object using a tracking component of the green-screenobject, instead of identifying the green-screen object according to itscolor or shape.

The system receives an initial image (step 606). The initial image wascaptured by a capturing component of one of the devices. In someimplementations, the system can receive two or more initial imagescaptured by a capturing component of a stereoscopic device, wherein thetwo or more initial images depict the real-world physical scene at thesame time point from slightly different angles.

Optionally, the system can receive position data characterizing the oneor more devices and/or one or more green-screen objects (step 608). Thesystem can use the new position data to update the maintained modelscharacterizing the position of the devices and/or green-screen objects.

Optionally, the system can receive one or more context images of thereal-world physical scene (step 610). The context images can be capturedby a camera, e.g., the camera 570 in FIGS. 5A and 5B, and can be used bythe system to replace the green-screen object with the virtual object inthe received initial images. The context images can provide informationabout what is behind the green-screen objects relative to the devicethat captured the initial image.

The system determines a position of the green-screen object in thephysical scene (step 612). The position of the green-screen object caninclude coordinates defining a location of the green-screen object inthe scene. The system can also determine associated data that identifiesan orientation and size of the green-screen object in the initial image,relative to a coordinate system of the scene. This model of thegreen-screen object can be in the same coordinate system as the model ofthe virtual object.

In some implementations, the system can determine the position of thegreen-screen object in the physical scene using multiple images of thephysical scene taken by the same device across different time points,using a motion parallax algorithm. That is, the system can detect thegreen-screen object for each image, where the green-screen object isdepicted from a slightly different angle at each time point. Then, thesystem can infer the position of the green-screen object in the physicalscene according to a coordinate system using a motion parallaxalgorithm, e.g., using one of the algorithms described in A Survey ofMotion-Parallax-Based 3-D Reconstruction Algorithms, Lu et al., DOI:10.1109/TSMCC.2004.829300.

In some such implementations, the system can identify the green-screenobject in the initial image by identifying pixels of a particular color,e.g., green or blue.

In some other such implementations, the system can identify thegreen-screen object in the initial image by identifying a particularshape in the initial image, e.g., using computer vision techniques. Insome other implementations, the system can determine the location andheading of the green-screen object relative to the device in thephysical scene using the maintained position data of the green-screenobject and devices (see step 604). In these situations, the systemrecognizes where within the image frame the green-screen region is andcorrelates that to the corresponding pixels of the image frame. Onlythese pixels then get replaced with the appropriate projected renderedvirtual scene or objects. The replaced pixels together with the originalimage pixels comprise the new rendered image to be displayed on the oneor more mono or stereo displays.

In the implementations where the system receives two or morestereoscopic initial images, the system can process the initial imagestogether to determine the position of the green-screen object. Thesystem can use differences between the images, as well as the distancebetween the respective cameras that captured the images, e.g., thedistance between the first camera 534 a and the second camera 534 b inFIGS. 5A and 5B, to geometrically determine the position of thegreen-screen objects in the physical scene. Using two stereoscopicimages allows the system to determine the projection; that is, thelocation and heading of the green-screen object more precisely than ifthe system only had access to a single monoscopic initial image; inparticular, the system in an implementation is able to determine thedepth of the green-screen object and the distance of the green-screenobject from the device more precisely.

After determining the position of the green-screen object, the systemcan update the maintained model characterizing the position of thegreen-screen object.

The system projects the virtual object to be rendered over green-screenobject in the received initial image to generate a processed image (step614). Using the geometry determined in step 612, the system can replacethe green-screen object in the initial image with the virtual object sothat the virtual object in the generated processed image has the correctposition, orientation, and size.

For example, the system can correlate each of the dimensions of themodel and the associated data of the virtual object with a correspondingdimension of the physical green-screen object, so that a particularposition and/or orientation of the green-screen object in the physicalscene can be mapped one-to-one to a particular position and/ororientation of the virtual object. As a particular example, the model ofthe virtual object can define a particular direction as the “forward”direction; similarly, the system can determine a “forward” direction ofthe identified green-screen, e.g., according to the shape of the objectfrom the perspective of the captured image. Then, the system cantransform the model, using the associated data, so that the two“forward” directions are aligned.

The virtual object is constrained to be strictly within the green-screenobject in the model of the scene, and therefore the displayed pixelsthat were the pixels of the green-screen object are the pixels that arereplaced with the rendered virtual object of scene. That is, thedepiction of the virtual object in a processed image cannot extendbeyond the portions of the initial image that depicted the green-screenobject. In some implementations, the model of the virtual object can bescaled to fit within the model of the green-screen object, i.e., thedimensions of the model of the virtual object that define the size ofthe virtual object are decreased, to scale, until the model of thevirtual object is strictly inside the model of the green-screen objectin the common coordinate system. In some other implementations, theportions of the model of the virtual object that extend beyond the modelof the green-screen objet can be clipped, so that the clipped model ofthe virtual object fits strictly inside the model of the green-screenobject.

In the implementations where the system receives two or morestereoscopic initial images, the system can generate two processedimages where the projection based on the perspective or position of thevirtual object is slightly different, according to the distance betweenthe respective cameras that captured the initial images.

The system displays the processed image on the display component of thedevice that captured the initial image (step 616). In implementationswhere the system received two stereoscopic initial images, the systemcan display each processed image on the respective eye's display, ordisplay both images on a single display; e.g., the system can displayboth images consecutively on a time-sequential stereoscopic display.

A computer generated background image replacing an identified greenscreen region, where the green screen region is within a camera captureddisplay window, where the computer generated background image is from a3D scene model whose 3D rendering may be positioned/oriented in space tothe mapped identified green screen region, thereby replacing the greenscreen region with the rendering being a projection rendering, based onthe detected position/orientation to a user's capture/viewing devicewhich renders the scene model, and where the capture mechanism is acamera (or other capture mechanism) that captures the physical spacebefore the capture/viewing device, where the 3D scene model is mapped toin position/orientation to the physical space captured by the capturemechanism.

It should be noted that the above-described embodiments are exemplaryonly, and are not intended to limit the invention to any particularform, function, or appearance. Moreover, in further embodiments, any ofthe above features may be used in any combinations desired. In otherwords, any features disclosed above with respect to one method or systemmay be incorporated or implemented in embodiments of any of the othermethods or systems.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

We claim:
 1. A method comprising: receiving a stereoscopic video imageof a physical scene captured by a camera, the physical scene comprisingan object having a tracking component; storing data providing a modeldefining (i) dimensions of a three-dimensional green-screen region and(ii) a location and/or orientation of the three-dimensional green-screenregion relative to the object; receiving, from the tracking component,data identifying a location of the object; identifying athree-dimensional region of space in the physical scene for thethree-dimensional green-screen region based on (i) the location receivedfrom the tracking component and (ii) the data defining dimensions of thethree-dimensional green-screen region and the location and/ororientation of the three-dimensional green-screen region relative to theobject; identifying an eyepoint pair in the physical scene; identifying,for each eyepoint of the eyepoint pair, a position and/or orientation ofthe three-dimensional green-screen region relative to the eyepoint; andrendering a modified stereoscopic video image onto a display surface inwhich the three-dimensional green-screen region is replaced with adepiction of a virtual three-dimensional object, the renderingcomprising: obtaining a three-dimensional model of the virtualthree-dimensional object, determining a three-dimensional boundary ofthe three-dimensional green-screen region, determining, for eacheyepoint of the eyepoint pair, a respective different perspective of thevirtual three-dimensional object based on (i) the three-dimensionalmodel of the virtual three-dimensional object and (ii) the positionand/or orientation of the three-dimensional green-screen region relativeto the eyepoint, and for each eyepoint of the eyepoint pair, performinga projection rendering of the three-dimensional model of the virtualthree-dimensional object based on the determined perspective of theeyepoint such that the depiction of the virtual three-dimensional objectis constrained to be within the determined three-dimensional boundary ofthe three-dimensional green-screen region.
 2. The method of claim 1,wherein: the method further comprises storing data providing a scenemodel that is a three-dimensional model of the physical scene, andidentifying the three-dimensional region of space in the physical scenefor the three-dimensional green-screen region comprises: updating thescene model using the location received from the tracking component, andidentifying the three-dimensional region of space in the physical scenebased on the updated scene model.
 3. The method of claim 1, wherein, foreach eyepoint of the eyepoint pair, performing a projection rendering ofthe three-dimensional model of the virtual three-dimensional objectcomprises: identifying a portion of the stereoscopic video image thatdepicts the three-dimensional green-screen region based on the positionand/or orientation of the three-dimensional green-screen region relativeto the eyepoint; and performing the projection rendering onto theidentified portion of the stereoscopic video image based on thedetermined perspective of the eyepoint.
 4. The method of claim 1,wherein: the method further comprises receiving a context image of thephysical scene captured by a second camera, and rendering the modifiedstereoscopic video image comprises rendering, using the context image, asub-region of the three-dimensional green-screen region that (i) isoccluded in the stereoscopic video image from the perspective of atleast one eyepoint and (ii) is not occluded in the modified stereoscopicimage from the perspective of the at least one eyepoint.
 5. The methodof claim 1, wherein the model defines the dimensions of thethree-dimensional green-screen region according to a regional functionof the object.
 6. The method of claim 5, wherein the model defines thethree-dimensional green-screen region to be the three-dimensional regionof space extending outward in all directions from the object.
 7. Themethod of claim 1, wherein identifying, for each eyepoint of theeyepoint pair, a position and/or orientation of the three-dimensionalgreen-screen region relative to the eyepoint comprises: receiving, froma second tracking component, data identifying a location of the eyepointand/or a location of a display device that is configured to display themodified stereoscopic video image; and identifying the position and/ororientation of the three-dimensional green-screen region relative to theeyepoint based on (i) the location of the object received from thetracking component, (ii) the data defining dimensions of thethree-dimensional green-screen region and the location and/ororientation of the three-dimensional green-screen region relative to theobject, and (iii) the location of the eyepoint and/or the location ofthe display device received from the second tracking component.
 8. Acomputer program product comprising a non-transitory computer readablemedium having instructions for causing one or more processors to:receive a stereoscopic video image of a physical scene captured by acamera, the physical scene comprising an object having a trackingcomponent; store data providing a model defining (i) dimensions of athree-dimensional green-screen region and (ii) a location and/ororientation of the three-dimensional green-screen region relative to theobject; receive, from the tracking component, data identifying alocation of the object; identify a three-dimensional region of space inthe physical scene for the three-dimensional green-screen region basedon (i) the location received from the tracking component and (ii) thedata defining dimensions of the three-dimensional green-screen regionand the location and/or orientation of the three-dimensionalgreen-screen region relative to the object; identify an eyepoint pair inthe physical scene; identify, for each eyepoint of the eyepoint pair, aposition and/or orientation of the three-dimensional green-screen regionrelative to the eyepoint; and render a modified stereoscopic video imageonto a display surface in which the three-dimensional green-screenregion is replaced with a depiction of a virtual three-dimensionalobject, the instructions to render comprising instructions to: obtain athree-dimensional model of the virtual three-dimensional object,determine a three-dimensional boundary of the three-dimensionalgreen-screen region, determine, for each eyepoint of the eyepoint pair,a respective different perspective of the virtual three-dimensionalobject based on (i) the three-dimensional model of the virtualthree-dimensional object and (ii) the position and/or orientation of thethree-dimensional green-screen region relative to the eyepoint, and foreach eyepoint of the eyepoint pair, perform a projection rendering ofthe three-dimensional model of the virtual three-dimensional objectbased on the determined perspective of the eyepoint such that thedepiction of the virtual three-dimensional object is constrained to bewithin the determined three-dimensional boundary of thethree-dimensional green-screen region.
 9. The computer program productof claim 8, wherein: the non-transitory computer readable medium furthercomprises instructions for causing the one or more processors to storedata providing a scene model that is a three-dimensional model of thephysical scene, and the instructions to identify the three-dimensionalregion of space in the physical scene for the three-dimensionalgreen-screen region comprise instructions to: update the scene modelusing the location received from the tracking component, and identifythe three-dimensional region of space in the physical scene based on theupdated scene model.
 10. The computer program product of claim 8,wherein, for each eyepoint of the eyepoint pair, the instructions toperform a projection rendering of the three-dimensional model of thevirtual three-dimensional object comprise instructions to: identify aportion of the stereoscopic video image that depicts thethree-dimensional green-screen region based on the position and/ororientation of the three-dimensional green-screen region relative to theeyepoint; and perform the projection rendering onto the identifiedportion of the stereoscopic video image based on the determinedperspective of the eyepoint.
 11. The computer program product of claim8, wherein: the non-transitory computer readable medium furthercomprises instructions for causing the one or more processors to receivea context image of the physical scene captured by a second camera, andthe instructions to render the modified stereoscopic video imagecomprise instructions to render, using the context image, a sub-regionof the three-dimensional green-screen region that (i) is occluded in thestereoscopic video image from the perspective of at least one eyepointand (ii) is not occluded in the modified stereoscopic image from theperspective of the at least one eyepoint.
 12. The computer programproduct of claim 8, wherein the model defines the dimensions of thethree-dimensional green-screen region according to a regional functionof the object.
 13. The computer program product of claim 12, wherein themodel defines the three-dimensional green-screen region to be thethree-dimensional region of space extending outward in all directionsfrom the object.
 14. The computer program product of claim 8, whereinthe instructions to identify, for each eyepoint of the eyepoint pair, aposition and/or orientation of the three-dimensional green-screen regionrelative to the eyepoint comprise instructions to: receive, from asecond tracking component, data identifying a location of the eyepointand/or a location of a display device that is configured to display themodified stereoscopic video image; and identify the position and/ororientation of the three-dimensional green-screen region relative to theeyepoint based on (i) the location of the object received from thetracking component, (ii) the data defining dimensions of thethree-dimensional green-screen region and the location and/ororientation of the three-dimensional green-screen region relative to theobject, and (iii) the location of the eyepoint and/or the location ofthe display device received from the second tracking component.
 15. Asystem comprising one or more computers and one or more storage devicesstoring instructions that are operable, when executed by the one or morecomputers, to cause the one or more computers to perform operationscomprising: receiving a stereoscopic video image of a physical scenecaptured by a camera, the physical scene comprising an object having atracking component; storing data providing a model defining (i)dimensions of a three-dimensional green-screen region and (ii) alocation and/or orientation of the three-dimensional green-screen regionrelative to the object; receiving, from the tracking component, dataidentifying a location of the object; identifying a three-dimensionalregion of space in the physical scene for the three-dimensionalgreen-screen region based on (i) the location received from the trackingcomponent and (ii) the data defining dimensions of the three-dimensionalgreen-screen region and the location and/or orientation of thethree-dimensional green-screen region relative to the object;identifying an eyepoint pair in the physical scene; identifying, foreach eyepoint of the eyepoint pair, a position and/or orientation of thethree-dimensional green-screen region relative to the eyepoint; andrendering a modified stereoscopic video image onto a display surface inwhich the three-dimensional green-screen region is replaced with adepiction of a virtual three-dimensional object, the renderingcomprising: obtaining a three-dimensional model of the virtualthree-dimensional object, determining a three-dimensional boundary ofthe three-dimensional green-screen region, determining, for eacheyepoint of the eyepoint pair, a respective different perspective of thevirtual three-dimensional object based on (i) the three-dimensionalmodel of the virtual three-dimensional object and (ii) the positionand/or orientation of the three-dimensional green-screen region relativeto the eyepoint, and for each eyepoint of the eyepoint pair, performinga projection rendering of the three-dimensional model of the virtualthree-dimensional object based on the determined perspective of theeyepoint such that the depiction of the virtual three-dimensional objectis constrained to be within the determined three-dimensional boundary ofthe three-dimensional green-screen region.
 16. The system of claim 15,wherein: the operations further comprise storing data providing a scenemodel that is a three-dimensional model of the physical scene, andidentifying the three-dimensional region of space in the physical scenefor the three-dimensional green-screen region comprises: updating thescene model using the location received from the tracking component, andidentifying the three-dimensional region of space in the physical scenebased on the updated scene model.
 17. The system of claim 15, wherein,for each eyepoint of the eyepoint pair, performing a projectionrendering of the three-dimensional model of the virtualthree-dimensional object comprises: identifying a portion of thestereoscopic video image that depicts the three-dimensional green-screenregion based on the position and/or orientation of the three-dimensionalgreen-screen region relative to the eyepoint; and performing theprojection rendering onto the identified portion of the stereoscopicvideo image based on the determined perspective of the eyepoint.
 18. Thesystem of claim 15, wherein: the operations further comprise receiving acontext image of the physical scene captured by a second camera, andrendering the modified stereoscopic video image comprises rendering,using the context image, a sub-region of the three-dimensionalgreen-screen region that (i) is occluded in the stereoscopic video imagefrom the perspective of at least one eyepoint and (ii) is not occludedin the modified stereoscopic image from the perspective of the at leastone eyepoint.
 19. The system of claim 15, wherein the model defines thedimensions of the three-dimensional green-screen region according to aregional function of the object.
 20. The system of claim 19, wherein themodel defines the three-dimensional green-screen region to be thethree-dimensional region of space extending outward in all directionsfrom the object.
 21. The system of claim 15, wherein identifying, foreach eyepoint of the eyepoint pair, a position and/or orientation of thethree-dimensional green-screen region relative to the eyepointcomprises: receiving, from a second tracking component, data identifyinga location of the eyepoint and/or a location of a display device that isconfigured to display the modified stereoscopic video image; andidentifying the position and/or orientation of the three-dimensionalgreen-screen region relative to the eyepoint based on (i) the locationof the object received from the tracking component, (ii) the datadefining dimensions of the three-dimensional green-screen region and thelocation and/or orientation of the three-dimensional green-screen regionrelative to the object, and (iii) the location of the eyepoint and/orthe location of the display device received from the second trackingcomponent.